BlankSequence

__ (two _ characters) or BlankSequence[]

is a pattern object that can stand for any sequence of one or more Wolfram Language expressions.

can stand for any sequence of one or more expressions, all of which have head h.

Examples

A function definition that works with any nonzero number of arguments:

Wolfram Language code: f[x__] := Length[{x}]

Wolfram Language code: {f[x, y, z], f[]}

A function definition that works with any nonzero number of real arguments:

Wolfram Language code: f[x__Real] := Plus[x] / Length[{x}]

Wolfram Language code: {f[1., N[Pi]], f[1, N[Pi]]}

A function definition for a symbol followed by any nonzero number of integer arguments:

Wolfram Language code: f[x_Symbol, p__Integer] := Apply[Plus, x ^ {p}]

Wolfram Language code: {f[x, 1, 2], f[0, 1, 2], f[x, 1, 2.]}

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